1. Field of the Invention
The present invention relates to a pneumatic tire having a rib pattern including a plurality of land portions continuing substantially circumferentially of the tire and, more specifically, to a pneumatic tire having a rib pattern which improves wear resistance, improves stone-nipping resistance, prevents generation of cracks on the groove bottom, and secures braking property and driving property when having worn.
2. Description of the Related Art
Uneven wear of a pneumatic tire having a rib pattern mainly includes step wear starting from grounded ends in the direction of axis of rotation of the tire and river wear generating at rib ends on both sides of a main groove, and there is a problem in the related art that particularly the river wear which is uneven wear occurring locally in the circumferential direction at the rib ends tends to occur.
An effective countermeasure for the step wear is to prevent the uneven wear by providing circumferential narrow grooves in the vicinity of the grounded ends of the shoulders of a tread surface. Also, in order to cope with the river wear, a technology to form a number of short sipes arranged along the rib ends circumferentially of the tire so as to extend widthwisely of the tire and open at one end into the main groove is known.
On the other hand, the rib pattern has a problem that it tends to nip stones in the groove, and the nipped stone is firmly held in the groove and cannot come out easily. In order to prevent the stone-nipping in the circumferential groove, JP-A-2000-185525 and JP-A-2001-55013 propose provision of a plurality of narrow ridges or projections having a circumferential length of 5 to 100 mm, at regular intervals of 0.5 to 20 mm on the bottom surface of the circumferential grooves on the tire tread portion.
As a tire which achieves both water drainage property and noise resistance of the tire, JP-A-2-212203 discloses a tire having a rib pattern including a groove top having a step extending circumferentially of the tire substantially in parallel to the tread surface at a level of a predetermined depth from the tread surface and a groove bottom formed radially inwardly from the groove top, in which the groove bottom is serpentined circumferentially of the tire.
The linear main groove of the rib pattern generally formed into a V-shape in cross-section for preventing the stone-nipping, and hence the proximal portion of the rib is smaller in width in comparison with a ground contact surface. Therefore, when the rib comes into contact with the ground, the rib is deformed by vertical force on load and groove walls are bulged toward the center of the groove, the groove walls tend to bulge on the side of the ground contact surface having a smaller width in comparison with the groove bottom, and hence the uneven wear can easily occur due to the movement of rubber.
The tire attached to a steering wheel is subject to a relative slippage with respect to the road surface on the sides of the ground contact ends of the shoulders which are relatively smaller in diameter in relation to the curvature of the surface of the ground contact surface of the tire. Therefore, uneven wear tends to occur on the shoulder sides in comparison with the center side.
The rib pattern has the problem of the above-described stone-nipping, and also a problem in the braking property and the driving property on the wet road surface. Therefore, even when the short sipes are provided as described above, there remains a problem of lowering of the braking property on the wet road on and after the medium phase of wear.